Projection display devices that use zoom lenses, are known. Japanese Laid-Open Patent Application H10-268193, Japanese Laid-Open Patent Application 2000-292701, and Japanese Laid-Open Patent Application 2001-004919 disclose five group zoom lenses in which three lens groups move to control zooming.
In general, zoom lenses for projection display devices need to provide a bright image based on the use of a liquid crystal image source of limited brightness. Additionally, the zoom lens needs to be compact to achieve the desired small size of the projection display device. In recent years, there has been a demand to be able to project the image from a short distance to a large screen by using a wider wide-angle projection lens in the projection display device. Furthermore, a larger zooming ratio is being demanded.
In addition, for use in a projection display device with a liquid crystal image source, a prerequisite for the projection zoom lens is that it be telecentric or nearly telecentric on the reducing side, that is, the side where the liquid crystal image source is present. Furthermore, it is necessary to provide an adequate back focus distance in order to insert an optical system for color synthesis in projection display devices (or for color separation in color separation imaging systems) between the lens system and the imaging plane.
Also, higher definition images have been increasingly demanded in the projection display device, as well as further improvement in various aberrations in the middle range of zooming with three lens groups moving for zooming, as described above.
Improvements have been sought by the use of six-group zoom lenses with four moving lens groups as disclosed in Japanese Laid-Open Patent Application 2001-350094, Japanese Laid-Open Patent Application 2001-235679, and Japanese Laid-Open Patent Application 2001-350096. However, none of these three published Japanese applications suggest improvements to the various aberrations in the middle range of zooming. Additionally, the zoom lenses of these three published Japanese applications are unsatisfactory for at least the following reasons. In Japanese Laid-Open Patent Application 2001-350094 and Japanese Laid-Open Patent Application 2001-235679, a maximum angle of view of approximately 52xc2x0 is achieved, but the correction of chromatic aberration is unsatisfactory. In Japanese Laid-Open Patent Application 2001-350096, a maximum angle of view of about 51xc2x0 is achieved, but the overall size of the projection optical system is too large. In addition, the fifth lens group from the enlarging side moves nearer the reducing side during zooming toward the telephoto end. That requires more space for moving the fifth lens group, which, in turn, makes it difficult to increase the zoom ratio while maintaining compactness of the projection display device.
The present invention relates to a zoom lens that includes six lens groups, four of which move for zooming, and in which various aberrations are favorably corrected. The required space for lens group movements during zooming is small, the zoom lens is compact, and has a large zoom ratio with a wide-angle of view and a bright image. The present invention further relates to a projection display device, such as a projection television, that uses such a zoom lens with, for example, a liquid crystal image source. Additionally, the present invention relates to cameras that use such a zoom lens for imaging onto an image detector such as a CCD, a camera tube element, photographic film, or similar devices.